High in the backcountry hills, six rafters thread their way along a narrow mountain stream. Ferns, vines, grasses and trees close in above the slick rock and the sound of rushing water is broken only by the click of paddle against gorge wall or boulder
The rafters are each sitting in a large inner tube ideal for traversing these waters - at times dark and deep, at others shallow and peppered with white-water rapids. Elsewhere the procession might be punctuated by yells of delight and excitement as the adventurers tumble downwards, pushed this way and that by the currents and eddies.
But here, today, it's a silent crew. They're watching. Listening. Scanning ledges, banks and overhangs for signs of their quarry, the beautifully camouflaged blue duck or whio.
Classified as "endangered" by the International Union of Conservation for Nature and as "nationally vulnerable" by New Zealand's Department of Conservation, the whio - named for the high-pitched whistle made by the male - has been severely impacted by exotic predators such as stoats. Once widespread throughout New Zealand, its population is now severely fragmented and, despite significant conservation efforts, remains in decline.
Concern for the long-term survival of the species is growing as its already-reduced range continues to contract. The whio population is increasingly dominated by males and chick counts are falling. It's estimated that about 640 pairs live in the North Island with less than 700 pairs in the South Island.
It's late December, smack bang in the middle of the whio breeding season, and the rafters are exploring the catchments of the Waiau and Te Hoe rivers bordering a 6120-hectare expanse of native bush in inland Hawke's Bay. This is the Maungataniwha Native Forest managed by the Forest Lifeforce Restoration Trust.
The FLR Trust was established in 2006 to help restore threatened species of fauna and flora, and ngahere mauri (forest lifeforce), to native forests owned by businessman Simon Hall.
The area was recently classified a whio recovery site by DoC. It found 19 breeding pairs along 41 kilometres of waterway. It also found 13 single ducks and 29 juveniles along the same stretches of water and concluded that whio are thriving in the area.
It's important to the FLR Trust that, in addition to adult population figures, it gets a good idea of how many chicks are being produced.
"This lets us know how effective our pest eradication and other protection initiatives are," says FLR trustee and forest manager Pete Shaw. He's leading the string of rafters and a notebook secured around his neck is littered with grid references and observations from sightings during today's float.
Whio numbers in the Maungataniwha block now appear to exceed the population density of many other North Island sites. South Island sites are naturally less dense so are not included in comparisons.
DoC is working with Genesis Energy on the Whio Forever project - a five-year, $2.5 million dollar programme that will double the number of secure breeding sites around the country.
The project classifies North Island sites it monitors as either security sites or recovery sites. The former are priority whio breeding areas and there are four across the North Island.
Recovery sites receive less funding but often have some form of stoat control. As with the FLR Trust's activities at Maungataniwha, stoat control at several recovery sites is funded and operated by private land owners.
At 0.47 pairs per km the Maungataniwha whio population density is approximately half the average of North Island security sites but more than double the average density of recovery sites. Similar whio densities at the FLR Trust's nearby Pohokura property and informal counts at Maungataniwha, such as this one, indicate that relatively substantial populations are likely to exist across the southern Whirinaki and Te Urewera ranges.
The FLR Trust, which recently gained $107,500 funding from the Genesis Energy Whio Forever project, hopes these results indicate at least a partial plateau in the decline of whio across inland Hawke's Bay.
The group rounds a bend in the river and one of the party signals a stop. Ahead, their silhouettes etched against the silvery disturbance of a shallow rapid, are two ducks.
"A breeding pair," Shaw says. "See how the male is scraping the rock? His upper bill has a thick fleshy 'lip' on it that lets him scrape insect larvae off rocks without wearing it down."
This is one of several unique evolutionary features that have resulted from the geographical isolation of this bird. Others include a streamlined head and large webbed feet that have helped the whio to feed in fast-moving water and become a "river specialist". It is the only member of its genus and has no close relative anywhere in the world.
Maungataniwha's close proximity to the Whirinaki security site, a little more than 15km away in a straight line, underscores its appeal as a whio recovery site.
Juveniles are easily able to travel between the two, assisting the group's long-term goal of establishing safe "stepping stones" between security sites.
According to DoC, the area's whio are benefitting from the extensive predator trapping undertaken by the FLR Trust in support of its Maungataniwha Kiwi Project, one of the most prolific and successful kiwi conservation initiatives in the country.
Just as the rafters prepare to continue down the gorge the female duck issues her tell-tale call - a guttural rattle. There's a movement among the roots of an overhanging tree and the six humans freeze into stillness once again.
From beneath the overhang emerge three whio ducklings. And Shaw's smile is a mile wide.